Telecommunications RF signals carry ever more information, coded by virtue of various types of modulation (OFDM, 16-QAM, etc.) that cause, in particular, large and rapid variations in their envelopes. Given the low electrical efficiencies of conventional RF amplifiers for amplifying these signals exhibiting complex modulations, various amplifier architectures have been proposed and developed that allow better electrical efficiencies of amplification to be obtained and a good level of linearity to be retained.
One technique for improving the electrical efficiency of amplification consists particularly of dynamically managing the polarization of a radiofrequency amplifier according to the variations in the envelope of the signal to be amplified. “Indeed, envelope tracking” is spoken of in the technical literature.
Systems based on this principle are composed, inter alia, of voltage modulators, themselves composed of switched-mode power supplies, whose passband bandwidth must be at least as wide as that of the envelope of the RF signals to be amplified.
A key element in these switched-mode power supplies is the power switching cell, which allows a power signal to be switched at its output according to a low-power control signal applied to its input. In the field of interest, the power switching cell must, more particularly, satisfy two criteria: high switching speed and low electrical consumption.
In order to meet these needs, transistors referred to as RF transistors (i.e. generally used for RF or microwave frequency applications) may be used. High-electron-mobility transistors (HEMTs), in particular HEMTs using GaN (gallium nitride) technology, are suitable for these types of applications in that they are capable of conducting large currents at high voltages (conferring thereon properties favorable for use in power circuits) while exhibiting intrinsic parasitic capacitances that are low with respect to other technologies (allowing them to have high switching speeds).
Constructional Difficulties and Particularities of Fast Modulators
One of the main difficulties in producing high-speed switching circuits resides in achieving control of power transistors whose source potential is floating, i.e. not referenced to a fixed potential (generally ground): it varies between a potential close to zero and a potential corresponding to the level of the high voltage to be switched. Techniques for isolating the gate control by virtue of optocouplers or isolation transformers do not appear to be suitable for these high-frequency switching applications due to their relative slowness. Optocoupler switching cells and associated problems are, for example, described in documents U.S. Pat. No. 5,514,996 A and JPS62296617A. Furthermore, a notable particularity of many RF field-effect transistors and, in particular, of most GaN HEMTs, is that they exhibit a “depletion” operating mode. This means that their (N-type) channel is open when the voltage applied between their gate and their source is zero. Under these conditions, if the voltage between drain and source is non-zero, a non-zero current may flow between drain and source. In order to turn off such a transistor, it is necessary to deplete its channel of free charges, which is achieved by applying a negative gate-source voltage that is lower than or equal to the voltage referred to as the transistor pinch-off voltage, denoted by Vp (this voltage Vp is therefore negative). “Normally on” transistors are spoken of in the technical literature. The use of normally on transistors, even though they pose no particular difficulty, is not common for switching applications and requires the known architectures to be adapted to this specificity.
The gate bias control of a power transistor must thus be designed to both:
be suited to the normally on specificity of numerous RF field-effect transistors; and
allow high-speed switching, without risking transistor breakdown, i.e. with a gate voltage that varies in a well-controlled manner with respect to the floating source voltage.
Furthermore, electrical losses from the switch must be as low as possible:
by switching (passing from the ON state to the OFF state and from the OFF state to the ON state) as rapidly as possible in order to limit switching losses, which is made possible by the choice of technology of components and by producing a suitable gate control circuit;
by limiting conduction losses: by choosing transistors having the lowest possible resistances in the on state, and by limiting their output leakage currents in the off state, when their maximum output voltage is close to the supply voltage.